Because I have a few big sand dollars myself, I will include pictures of the ones in my collection to highlight the different types of sand dollars you may find while beach-combing.
I have three whole sand dollars, although the little one is slightly broken. We may call them shells, but they are not. They are ‘tests’ and are really the outer skeletons of the creature who made them. They are really echinoderms, like the sea urchin, sea cucumber and sea star (starfish). And they don’t start off white. Once the animal dies, the sand wears off their fuzzy outer coating, and the sun bleaches them. In life, they are colorful and can be green, purple, or brown.
In my photo here on the left, is the large, four inch arrowhead. It is flat and has a series of lines on the backside. I can hear the “jaws” rattling around inside. Those jaws are what move to digest the sand dollar’s food.
There are different types of sand dollars. The keyhole sand dollar is similar to the arrowhead, but has five small holes. The notched sand dollar has holes along the edge as you can see from the image below taken from Wikimedia.
The holes are used in part for helping the creature sink into the sand, where it lives and feeds.
In my photo below you can see my arrowhead with the large ‘inflated sea biscuit‘ and small ‘sea cookie‘. Those are puffy and bloated looking compared to the other types. They are covered with dark brown fuzz when alive. I wrote ‘Sanibel 1990’ on the bottom of this big one. I didn’t find it along the beach, but purchased it at a shop when I was on Sanibel Island on vacation back in the 80’s.
The little sea cookie is very fragile, and is broken as you can see. Sand dollars are delicate and will break easier than most shells. If you find a whole one on the beach, wrap it in something to carefully bring it home to clean.
While I was moving these shells around to take these pictures, one of the doves (jaws), fell out of the sea cookie. It is pictured below.